1. Field of the Invention
The present invention relates to an electric disk brake in which to generate braking force with motor torque, and more specifically to an electric disk brake with a parking brake mechanism, and a lock device suitable for the electric disk brake.
2. Description of the Related Art
Conventionally, an electric disk brake has been provided with a caliper comprising: a pad pressing member pressing a brake pad; a motor; and a rotary-to-linear motion conversion mechanism converting rotary motion of a rotor of the motor into linear motion so as to transmit the motion to the pad pressing member (a piston). The pad pressing member is adapted to move forward according to the rotary motion of the rotor of the motor so as to generate braking force by pressing the brake pad to a disk rotor.
In addition to the electric disk brake discussed above, development of an electric disk brake provided with a parking brake has been expedited in recent years; this kind of the electric disk brake is additionally provided with a lock mechanism enabling to lock and unlock displacement of a pad pressing member in a braking release direction so as to retain braking force even after electric power to a motor ceases. For example, in an electric disk brake disclosed in Japanese Patent Application Unexamined Publication No. 2003-42199 (hereinafter reference 1), a rotor of a motor is provided with a ring-shaped rotational body, so that an engaging claw can be engaged with and disengaged from a claw portion of the ring-shaped rotational body by means of an actuator (solenoid). After the rotor of the motor is shifted to a braking position, the engaging claw is engaged with the ring-shaped rotational body so as to retain a braking condition in a non-power state.
In the electric disk brake with the parking brake, however, in case that the parking brake is applied at a high temperature where a brake pad is thermally expanded, pressing force, that is, thrust force of the pad pressing member is deteriorated due to shrinkage of the brake pad along with a subsequent temperature decrease. In traditional methods, when the deterioration of the thrust force is caused due to temperature variations, re-application of the parking brake by re-rotating the motor in a power-intensified direction, so-called re-clamp operation, has been conducted to deal with the above problem. See, for example, Japanese Patent Application Unexamined Publication No. 2006-232263.
However, in the electric disk brake with the parking brake disclosed in the reference 1, the engaging claw is always energized with an energizing means in a direction being disengaged from (see FIGS. 9 and 10) or engaged with (see FIGS. 6 to 8) the claw portion of the ring-shaped rotational body. Accordingly, in case that the engaging claw is always energized in the disengaged direction, if the re-clamp operation is performed in condition that the actuator is broken down during activation of the parking brake, the engaging claw is disengaged from the claw portion of the ring-shaped rotational body, whereby it causes a condition that the parking brake can never be applied. On the other hand, in case that the engaging claw is always energized in the engaged direction, if the actuator is broken down in normal brakes, the lock mechanism will be automatically operated, making vehicle movements unstable (fail-safe unavailable).